Thanks, Ben. The adjustable extension stems were used mostly on track bikes in the '30s, '40s, an '50s, but there's no reason they can't be used on road bikes. My understanding is that track bikes were often used by multiple riders and the adjustable stems combined with adjustable seatposts made bikes quickly adaptable to different riders. Most road bikes are used just by their owners, so once the stem extension length is dialed in there's no reason to change it.